Hydraulic conductivity of compressible clayey soil stabilized with kraft black liquor and composite cement

This study examined the hydraulic behavior and geochemical interactions of a compacted high-plasticity clayey soil (CH) and its mixtures with kraft black liquor (KBL) and composite cements CP II˗E˗32 and CP II˗F˗32 for landfill liner applications. Mixtures with 3% and 5% black liquor (by dry soil mass) were tested for hydraulic conductivity and chemical stability. Specimens were statically compacted in PVC molds (10 cm in diameter, 12 cm in height) at optimum water content and dry unit weight, based on standard Proctor tests. Percolation tests were conducted at 20°C under a hydraulic gradient of 15, with leachate analyzed for pH and cation concentrations via atomic absorption spectrophotometry. Cation exchange capacity (CEC) was also determined. Black liquor reduced the optimum moisture content and increased the maximum dry density, promoting clay dispersion and lowering hydraulic conductivity by up to 10 times. Mixtures with 3% black liquor and 5% liquor-cement CP II˗F˗32 achieved the lowest hydraulic conductivity (2.4–3.4 × 10^–10 m/s). In contrast, mixtures with CP II˗E˗32 exhibited higher and more variable conductivity, likely due to increased Ca²⁺ leaching, which induced particle flocculation. Black liquor addition increased pH and Na⁺ concentration, enhancing CEC. Cement CP II˗F˗32 further intensified these effects, increasing Al³⁺ concentrations without impairing clay dispersion. A strong correlation was found between the \([\text{Na}]/[\text{Ca}]\) ratio and hydraulic performance, with higher values associated with lower hydraulic conductivity. These results highlight the potential of soil-liquor-cement mixtures, particularly those with CP II˗F˗32, for landfill liners due to improved hydraulic conductivity and chemical stability.